The present invention relates to a bleed circuit for a backhoe boom cylinder which minimizes cylinder pressure during over-center movement of the boom while permitting the maintenance of sufficient cylinder pressure during other boom operations.
A popular feature on backhoes for several years has been the over-center boom. This arrangement permits an over-center movement of the backhoe boom to a transport configuration wherein the center of gravity of the backhoe is closer to the rear of the transport vehicle. An arrangement of this type is disclosed in U.S. Pat. No. 3,376,984, which is assigned to the assignee of the present invention.
A typical procedure for bringing the backhoe boom over-center is to retract the boom cylinder until the boom cylinder reaches its over-center position. Then, the boom cylinder control valve is reversed and the boom continues its motion to an over-center position. The boom cylinder is retracted to provide sufficient velocity to the boom so that it continues its movement in response to kinetic energy, even after the boom cylinder reaches its over-center condition where the effective movement arm of the boom cylinder is lost. As the boom cylinder crosses over-center, its starts to extend. Hydraulic fluid in the retract side of the cylinder is locked in by the control valve for the boom cylinder until it is reversed. Thus, high boom cylinder pressures can be generated if the boom cylinder control valve reversals are not timed correctly. Moreover, the circuit relief valve does not provide pressure reduction in this over-center condition because it is set high to govern the lift capacity of the backhoe dipper arm.
Examples of prior art are shown in U.S. Pat. Nos. 3,054,384; 3,129,720; 3,272,085; 3,398,650; 3,523,490; 3,613,503; 3,668,975; 4,164,122; and Re. 28,695. However, none of these prior art patents are directed to minimizing cylinder pressure during the movement of a backhoe boom and cylinder arrangement over-center while maintaining pressure for other boom operations.
Therefore, the present invention is directed to a circuit for reducing backhoe boom cylinder pressure during over-center movement of the backhoe boom. Further, the present invention is directed toward improving the controlability of the backhoe boom since exact timing for reversing the boom cylinder control valve becomes less critical. Moreover, the present invention is directed toward limiting the range of the boom position to that segment of the total travel arc which is most useful for lifting purposes.